The production of energy from fossil fuels such as coal, oil and natural gas generates large amounts of gaseous, liquid and solid wastes, including coal fly ash, which typically contains various toxic heavy metals. It is estimated that today more than 600 million tons of coal ash are produced annually, about 500 million tons (75-80%) in the form of airborne fly ash. The industrial utilization of fly ash worldwide today ranges from a minimum of 3% up to a maximum 57%, with an average of 16% of the total fly ash produced. A large amount of coal ash is still disposed of in landfills. Two different techniques are used for ash disposal. In the wet disposal technique, large quantities of fly ash are collected as wet slurry and disposed of in ash ponds. The second technique involves disposing of the ash in dry form as ash mounds.
The disposal of coal ash in ponds and landfills creates environmental problems, including the leaching of heavy and trace elements into ground and surface water. Some of these elements are toxic and cause numerous diseases in plants, animals and humans. Trace and heavy metals from different fly ashes into the environment, including arsenic, barium, beryllium, cadmium, chromium, copper, lead, selenium, vanadium and other hazardous materials can leach from fly ash into the environment. Because of its heavy and trace element content, coal fly ash is an industrial byproduct that is recognized as an environmental pollutant.
Another byproduct of coal-fired power plants is SOx/NOx in flue gas, which can have serious negative impact on trees and plants. The abilities of soils in resisting and buffering acidity are determined by the thickness and composition of the soil along with the characteristics of bedrock beneath. Acid rain resulting from flue gas emission damages trees and plants by damaging their leaves, reducing the availability of nutrients, or increasing exposure to harmful substances in the soil. Therefore, the acidic gases in flue gas need to be removed. Conventional SOx/NOx removal methods use lime or limestone; however, the kinetics of the reactions between calcium materials (lime/limestone) and SOx/NOx is slow. Sodium based materials (Na2CO3NaHCO3/trona) can be used to overcome some shortcomings of calcium based SOx/NOx removal agents. The difficulty associated with sodium based SOx/NOx removal materials is that some trace elements or heavy metals, especially Arsenic (As) and Selenium (Se) in fly ashes become more leachable.
A simple, cost-effective approach for the remediation of fly ash associated with conventional lime-based desulfurization has not been available, since not all of the non-biodegradable heavy metals can be simultaneously removed under the same conditions. Increasing numbers of coal-fired power plants are using trona for desulfurization; however, the carbonate and bicarbonate introduced into the process by trona makes the heavy metals more leachable and difficult to remove.
Reducing the amount of fly ash released into the environment by taking advantage of its cementitious or binding characteristics for use as a construction material; the chemical remediation of fly ash; exploiting the high alkalinity of fly ash for use as a soil amendment; and removing leachable trace elements using different sorbents, have been proposed for addressing trona-associated coal fly ashes. Due to its high efficiency, ease of operation and the low cost and wide availability of sorbents, adsorption generally has been considered to be a promising technology; however, conventional adsorption technology does not work well such fly ashes, because of the complexity of its leachates.
The concentrations and types of heavy metals in fly ashes vary from one coal to another, such that methods for removal of heavy metals from fly ash likewise vary. Ion exchange, chemical precipitation, reverse osmosis, and solvent extraction have long been studied by many researchers. Nonetheless, it remains difficult to develop a cost-effective method that can simultaneously remove all toxic elements, particularly selenium, from fly ash associated with the use of trona for desulfurization in coal fired power plants.